The present invention relates to a method for actuating a component of a drive train of a motor vehicle, wherein the component is mechanically prestressed into a safe state, and also relates to a control device for carrying out such a method.
In the field of drive trains for motor vehicles there is a trend for adjustable components to be activated by means of electronically actuated actuators. The components can be a part of the drive engine (for example a throttle valve), but are, in particular, components which are arranged downstream of the drive engine in the direction of the power flux, such as starter clutches, transmissions, differentials, etc.
The actuators in this context can be fluidic or electromechanical actuators.
Since more stringent demands are made of safety in motor vehicle drive trains, generally safety systems are provided, which, when faults occur in the electronic actuation, are intended to transfer the affected component or the drive train into a safe state. Depending on the component, it is desired or required here in terms of safety technology that the component be transferred into a safe state within a safety time period (process safety time). If the activation of the component can lead, for example, to stresses in the drive train, the transfer into a safe state can prevent grip being lost between a driven wheel and an underlying track surface.
A safe state can mean here, for example, that the component is opened with the result that drive power can no longer be transmitted via said component. However, in many cases the safe state is associated with closing a component. For example, in this way in many cases a loss of tractive force can be prevented, which loss could be problematic for safety under certain circumstances during an overtaking process (see, for example, DE 10 2004 018 962 B3). From this document it is known, when a type of fault occurs in a double clutch transmission, to keep that clutch via which a relatively high torque can be transmitted closed.
Document DE 10 2006 054 253 A1 proposes that when an interference signal occurs both clutches of a double clutch transmission are firstly opened, and when a safe basic state is reached pressure is applied again to one of the two clutches. Under certain circumstances, there is intervention into the actuation of the drive engine.
In the case of a processor fault, a control device for actuating the component is reset. This generally leads to the outputs of the control device being switched off. In such a case, the safe state is achieved by the mechanical prestress of the component. From document WO 2008/049606 A1 it is known to provide, independently of the control device, a piece of safety hardware which is connected to an actuator arrangement in parallel with the control device and is configured to supply, in the case of a fault state, the actuator arrangement with electrical power independently of the control device in order to transfer a component in the form of a separation clutch into a safe state (for example to open said component).
If the actuator arrangement has an electric motor in the event of a fault the mechanical prestress must consequently not entrain the electric motor. Instead, the safe state is achieved by reversing the direction of rotation of the electric motor which assists the mechanical prestress for reaching the safe state.